This invention relates to fibers that are flash-spun from blends of polymers that include two or more polyolefin polymers. More particularly, the invention relates to flash-spun plexifilamentary fibers comprised of a polymer blend that includes significant polyethylene and polypropylene components.
The art of flash-spinning strands of plexifilamentary film-fibriis from polymer in a solution or a dispersion is known in the art. The term "plexifilamentary" means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean film thickness of less than about 4 microns and with a median fibril width of less than about 25 microns. In plexifilamentary structures, the film-fibril elements are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form a continuous three-dimensional network.
U.S. Pat. No. 3,227,784 to Blades et al. (assigned to E. I. du Pont de Nemours & Company ("DuPont")) describes a process wherein a polymer in solution is forwarded continuously to a spin orifice at a temperature above the boiling point of the solvent, and at autogenous pressure or greater, and is flash-spun into a zone of lower temperature and substantially lower pressure to generate a strand of plexifilamentary material. U.S. Pat. No. 3,227,794 to Anderson et al. (assigned to DuPont) teaches that plexifilamentary film-fibrils are best obtained from solution when fiber-forming polymer is dissolved in a solvent at a temperature and at a pressure above the pressure at which two liquid phases form, which pressure is generally known as the cloud point pressure at the given temperature. This solution is passed to a pressure let-down chamber, where the pressure decreases below the cloud point pressure for the solution thereby causing phase separation. The resulting two phase dispersion of a solvent-rich phase in a polymer-rich phase is discharged through a spinneret orifice to form the plexifilamentary strand.
U.S. Pat. No. 3,484,899 to Smith (assigned to DuPont) discloses an apparatus with a horizontally oriented spin orifice through which a plexifilamentary strand can be flash-spun. The polymer strand is conventionally directed against a rotating lobed deflector baffle to spread the strand into a more planar web structure that the baffle alternately directs to the left and right as the web descends to a moving collection belt. The fibrous sheet formed on the belt has plexifilamentary film-fibril networks oriented in an overlapping multi-directional configuration.
European Patent Publication 645480 filed by Unitika Ltd. discloses a plexifilamentary fiber structure that is flash-spun from a solution of polyolefin and polyester polymers dissolved in methylene chloride. The polyolefins disclosed include polyethylene and polypropylene polymers and copolymers. The polyesters disclosed include polyethylene terephthalate and polybutylene terephthalate. The Unitika patent discloses that the mixing ratio (by weight) of the polyolefin to the polyester is from 5/95 to 95/5.
British Patent Specification 970,070 (assigned to DuPont) discloses nonwoven sheets made from fibers that were flash-spun from a blend of polyethylene and a minor amount of another polymer such as polyamide, polyvinyl chloride, polystyrene, or polyurethane. The patent suggests that a "blends of linear polyethylene and minor amounts of branched polyethylene, polypropylene, polybutylene, polyisobutylene, polybutadiene, polyvinyl chloride, or cellulose acetate" might be advantageous. However, the patent does not appear to disclose the actual flash-spinning of polyethylene and polypropylene blends.
Many improvements to the basic flash-spinning process have been reported or patented over the years. An alternative process for flash-spinning a plexifilamentary strand according to which a mechanically generated dispersion of melt-spinnable polymer, carbon dioxide and water under high pressure is flashed through a spin orifice into a zone of substantially lower temperature and pressure to form a plexifilamentary strand is disclosed in U.S. Pat. No. 5,192,468 to Coates et al. (assigned to DuPont). Flash-spinning of polyethylene to produce non-woven sheets is practiced commercially and is the subject of numerous patents, including U.S. Pat. No. 3,851,023 to Brethauer et al (assigned to DuPont).
The commercial application for flash-spinning has been primarily directed to the manufacture of sheets of bonded polyethylene plexifilaments. Polyethylene is an ideal polymer for flash-spinning. It can be flash-spun into a strong well fibrillated plexifilament over a wide range of processing conditions. However, its melting point is relatively low (.about.140.degree. C.), and therefore it is not suitable for applications where end use temperatures are 140.degree. C. or higher. One such application is steam sterilizable sterile packing, and CSR (i.e., central storage room) wraps used in the hospitals for steam sterilization. Polypropylene, on the other hand, has a higher melting point (165.degree. C.) that is above the temperatures used during steam sterilization. However, polypropylene is more difficult to flash-spin than polyethylene and as-spun fibers are not as strong. In addition, polypropylene requires substantially higher spin temperatures than polyethylene.
There is a need for a flash-spun product that enjoys the strength and ease of processing associated with polyethylene, but that can withstand higher end use temperatures.